Mic cable too short?

You could use an extension cable like this one:  http://www.mcmelectronics.com/product/24-6265, but we advise against using extension cables longer than 25’. Because the mic signal is lower level than line-level audio signals, there will be significant high frequency loss in the cable. That loss will make the mic "think" that your speakers have decreased high frequency response and the result will be to incorrectly boost the high frequencies. 

Your best option is to temporarily install the AVR in the room and then move it after calibration.

I have the same problem. I cannot move the AVR to the same room because 4 of the speakers are in wall and in ceiling. I have an Onkyo 5500 amp and PSC5508 A/V Controller. Is the suggestion above the only approach? Is there a way to determine whether there is a high frequency issue and/or correct for it.

Hi Lee,

I'm afraid this is the only approach I know of.  We have already determined that longer cables will diminish the high frequencies too much because the mic level signal is much lower. 

Thanks Chris. Just to make sure i am clear, are you saying that any extension to the mic cord will create the problem and that the only way to get good Audyssey balancing is to move the AVR to the room?

or can i use a 15-25' extension?

You can use up to a 25' extension as mentioned above.